J. Am. Chem. SOC.1993,115, 4830-4843
4830
Chemistry in Cages: Zeolite-Entrapped Hexairidium Cluster Catalysts S. Kawi, J.-R. Chang, and B. C. Gates' Contribution)from the Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received August 5. 1992. Revised Manuscript Received October 30, 1992
Abstract: Iridium carbonyl clustersin the cages of NaY zeolite were prepared from adsorbed [Ir(CO)z(acac)] . Reductive carbonylation of this precursor in CO at 1 atm and 50 OC gave [Ir4(C0)1z],which was converted in CO at 125 OC and 1 atm to the isomer of [Ir6(CO)16] with edge-bridging ligands and in CO Hz at 250 OC and 20 atm to the isomer of [Ir6(CO)16] with face-bridging ligands, both identified by infrared and extended X-ray absorption fine structure (EXAFS) spectra. The zeolite containing the latter catalyzed the CO hydrogenation reaction at 20 atm and 250 OC, giving a high selectivity to propane and being stable for a number of days in a flow reactor. Either isomer of [Ir6(CO)16] in the zeolite could be decarbonylated in H2 at 300 OC and 1 atm and recarbonylated to give either isomer of [Ir6(CO)16]. The decarbonylated cluster in the presence of Hz is inferred from EXAFS spectra to be predominantly Ir6 with an octahedral structure and an average Ir-Ir bond distance of 2.71 A. These results illustrate the potential for preparation of supported metal clusters with controlled nuclearities and open the door to precise characterization of the structures and properties of uniform, highly dispersed supported metal catalysts.
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Introduction Nanostructures dispersed in the molecular-scale cages of crystalline aluminosilicates(zeolites) include metal1-' and metal carbonyl clusters," metal oxides?JOand metal sulfides.llJzThe encaged metal carbonyl clusters are catalysts with unusual selectivities for CO hydrogenation6J3-17and potential shapeselective catalysts.'* The encaged metal oxidesgJOand metal sulfidesl1.12are potential semiconductorsthat offer novel electronic and optical properties. The preparation of unique-nearly molecular-encaged nanostructures is often difficult, as mixtures often form, not only inside the cages but also outside the zeolite crystallites. To develop the chemistry in zeolite cages, it is essential to synthesize the encaged structures in high yields and to characterize them precisely; extended X-ray absorption fine structure (EXAFS) spectroscopyis well-suited to thesematerials, but so far there are only a few characterizations of them that include thorough EXAFS analyses.lS2l The samples chosen for this research are hexairidium clusters Address correspondence to this author at the Department of Chemical Engineering, University of California, Davis, CA 95616. (1) Jacobs, P. A. In Metal Clusters in Catalysis;Gates, B. C., Guczi, L., KnBzinger, H., Eds.; Elsevier: Amsterdam, 1986; p 357. (2) Kawi, S.;Gates, B. C. Catal. Lett. 1991, 10, 263. (3) Kawi, S.;Gates, B. C. J. Chem. Soc., Chem. Commun. 1991, 994. (4) Rode, E. J.; Davis, M. E.; Hanson, B. E. J . Catal. 1985, 96, 574. (5) Sheu, L.-L.; KnBzinger, H.;Sachtler, W. M. H. J. Am. Chem. Soc. 1989,111, 8125. (6) Kawi, S.;Gates, B. C. J . Chem. SOC.,Chem. Commun. 1992, 702. (7) Bergeret, G.; Gallezot, P.; Lefebvre, F. Stud. Surf. Sci. Catal. 1986, 28, 401. (8) Bergeret, G.; Gallezot, P.; Gelin, P.; Ben Taarit, Y.;Lefebvre, F.; Naccache, C.; Shannon, R. D. J. Caral. 1987,104, 279. (9) Ozin, G. A.; Ozkar, S.;Moller, K.; Bein, T. J. Am. Chem. Soc. 1990, 112,9575. (10) Ozin, G. A.; Malek, A.; Prokopowicz, R.; Macdonald, P. M.; Ozkar, S.;Moller, K.; Bein, T. Mater. Res. Soc. Symp. Proc. 1991, 233, 109. (11) Herron, N.; Wang, Y.;Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J . Am. Chem. Soc. 1989, 111, 530. (12) Moller, K.; Eddy, M. M.; Stucky, G. D.; Herron,N.; Bein, T. J. Am. Chem. Soc. 1989.11 1, 2564. (13) Zhou, P.-L.; Gates, B. C. J. Chem. Soc., Chem. Commun. 1989,347. (14) Zhou, P.-L.; Maloney, S.D.; Gates, B. C. J. Catal. 1991, 129, 315. (15) Lee, T. J.; Gates, B. C. Coral. Lerr. 1991, 8, 15. (16) Kawi, S.;Chang, J.-R.; Gates, B. C. J. Caral., in press. (17) Fraenkel, D.; Gates, B. C. J. Am. Chem. Soc. 1980,102, 2478. (18) Mantovani, E.; Palladino, N.; Zanobi, A. J. Mol. Carol. 1977,3,285. (19) Maloney, S.;Zhou, P.-L.; Kelley, M. J.; Gates, B. C. J. Phys. Chem. 1991, 95, 5409.
in zeolite Nay; preliminary infrared results' indicate that [Ir6(co)16] and decarbonylated clusters, suggested to be Ir6, have been formed in the zeolite supercages. Here we report the synthesis, characterization by infrared and EXAFS spectroscopies, and catalytic properties of the zeolite containing [Ir6(CO)l6]; we also report evidence that the cluster nuclearity of six is retained in the reversibledecarbonylationand carbonylationof the encaged clusters. RHdts Reactivity of [Ir(CO)z(acac)] in NaY Zeolite under CO at 1 atm. [Ir(CO)z(acac)] was adsorbed by NaY zeolite from hexane solution. Uptake of the organometallic species by the zeolite required some hours and was not complete even after 2 days of contacting. The infrared spectrum of the supernatant solution showed that some [Ir(CO)2(acac)] was still present, and there was no evidence of other metal carbonyls. Upon removal of the solvent from the black [Ir(CO)z(acac)]containing zeolite by evacuation, the solid became pale yellow. Reintroductionof the solvent gave a brown material, which became pale yellow again upon evacuation. The infrared spectrum of the pale yellow solid (Figure 1A) has two strong carbonyl bands, at 2080 and 2000 cm-I, indicating the existence of an iridium dicarbonyl species. A sample of the [Ir(CO)2(acac)]-containing zeolite in an infrared cell was exposed to flowing CO at 40 OC. After about 6 h, the spectrum (YCO: 21 14 w, 2070 s, 2029 m, sh cm-1) (Figure 1B) closely resembled that of [Ir4(CO)l2] in tetrahydrofuran (THF) solution (YCO: 21 10 vw,2068 vs, 2028 m, sh cm-1). The solid sample was light yellow, the color of [Ir4(CO)12]. EXAFS data2' are consistent with the identification of [Ir4(CO)lz]. When the temperature was increased to 125 OC, a bridging carbonyl band appeared at 1816 cm-I and increased in intensity as the terminal carbonyl bands shifted to higher frequencies. The spectrum attained a steady state after 4 h at 125 OC (Figure 1C); the sample was light yellow.3 The spectrum is almost the same as the THF solution spectrum of the isomer of [Ir6(co)16] that has four edge-bridging and terminal ligands,Zz except that the (20) Moller, K.; Koningsberger, D. C.; Bein, T. J. Phys. Chem. 1989.93, 6116. (21) Kawi, S.;Chang, J.-R.;Gates, B. C., to be published.
OOO2-7863/93/1515-4830$04.00/0 0 1993 American Chemical Society
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Figure 1. Infrared spectra taken during treatment of the initially prepared sample made from [Ir(CO)*(acac)] and NaY zeolite: (A) [Ir(CO)2(acac)] adsorbed in NaY zeolite; (B) after treatment in CO at 40 OC for 6 h; (C) after additional treatment in CO at 125 OC for 4 h; and (D) after additional treatment in CO at 175 "C for 1 h.
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Figure 2. Infrared spectra characterizing the reactivity of NaY zeolitesupported [Ir6(co)16] with edge-bridging ligands in 0 2 at 125 OC: (A) NaY zeolite-supported [Ir6(CO),6] with edge-bridging ligands; (9) after treatment in flowing H e / 0 2 at 125 "C for 15 min; and (C) after subsequent CO treatment at 125 OC for 30 min.
Reactivity of Zeolite-Supported Iridium Carbonyl Clusters in O2and in Air. A sample prepared from [Ir(C0)2(acac)]in NaY
frequencies of the terminal CO bands of the encaged cluster are zeolite was treated in the infrared cell in flowing CO at 125 OC shifted to higher frequencies and that of the bridging ligand is for 12 h to form the isomer of [h6(c0)16] with edge-bridging shifted to a lower frequency. Thus the iridium carbonyl is ligands (Figure 2A). The CO stream was then replaced by a tentatively assigned as [Ir6(co)16] with edge-bridging carbonyl stream of He containing 2% 02. The infrared spectrum ligands. A similar infrared spectrum has been observed for an immediately changed;the bands at 2085 and 18 16cm-l decreased iridium carbonyl in a zeolite and assigned incorrectly as [Irdand the bands at 2080 and 2000 cm-1 increased in intensity. After (CO) I 21 ,23 but this report appeared prior to the identification of about 15 min, the bands at 2085 and 18 16 cm-1 had disappeared, the isomer of [Ir6(co)16] with edge bridging co ligands. leavingtwo intense bands in the YCO region (Figure 2B),indicative With the sample still in the cell under CO at 1 atm, the of the oxidized sample. When this sample was treated in flowing temperature was raised to 175 OC. After 1 hat 175 OC, the band CO at 125 OC, the spectrum of the isomer of [Ir6(co)I6] with at 1816 cm-I decreased slightly in intensity, the terminal carbonyl edge-bridging ligands reappeared (Figure 2C). peaks shifted about 10 cm-I to higher frequencies, and a peak Another sample prepared from [Ir(C0)2(acac)] and NaY appeared at 1730 cm-1 (Figure 1D);the sample was still light zeolite was treated in the infrared cell in flowing CO at 125 OC yellow. The 1730-cm-1 band is characteristic of the isomer of for 12 h to form the isomer of [Ir6(C0)16] with edge-bridging [Ir6(co)16] that has four facebridging and 12 terminal ligand~?~3~~ ligands (Figure 3A). The sample was cooled to room temperature The spectra show that roughly 60% of the zeolite-encaged [Irsin flowing CO. The cell was then purged with NZfor 15 min to (CO)16] with edge-bridging ligands had been converted to [Irsremove CO gas. The spectrum of [Ir6(CO)16]was maintained. (CO) 161 with face-bridging ligands. At temperatures greater The sample was then exposed to air. The bridging band at 1816 than 240 OC, the infrared bands of the carbonyl species were cm-1 decreased in intensity, and the terminal bands shifted to broadened, and after >5 h at 250 OC, the infrared spectrum lower frequencies after the sample was exposed to air for 5 min showed a broad band at 2030 cm-1 characteristic of CO (Figure 3B). After the sample was exposed to air for 15 min, its chemisorbed on iridium meta1;26 the sample was then gray, and infrared spectrum was similar to that of [Ir4(CO)12](Figure the metal had evidently aggregated outside the zeolite pores. 3C). The isomer of [Ir6(C0)16] with edge-bridging ligands was Attempts to extract iridium carbonyls from samples containing reformed by treatment of the sample in flowing CO at 125 OC either of the two [Ir6(co)16] isomers with THF or with bisfor 30 min (Figure 3D). (tripheny1phosphine)iminium chloride, [PPN] [Cl], in THF were Catalytic CO Hydrogenation. All the Ir-containing samples unsuccessful. The supernatant solutions remained colorless and were found to catalyze CO hydrogenation at temperatures of had no carbonyl absorptions,consistentwith the stable entrapment 225-275 OC. Conversions were measured as a function of time of the clusters in the zeolite. on stream in a flow reactor. Catalytic activities are represented by rates of reaction calculated from low-conversion data and are (22) Garlaschelli, L.; Martinengo, S.; Bellon, P. L.; Demartin, F.; expressed in units of [mol of CO converted to hydrocarbons (mol Manassero, M.; Chiang, M. Y.; Wei, C.-Y.;Bau, R. J . Am. Chem.Soc. 1984, 106,6664. of total Ir.s)-I]. All the experiments reported here were conducted (23) Gelin, G.; Ben Taarit, Y.; Naccache, C. Proc. 7th Inr. Congr. Carol. with CO conversions between 0.1 and 2.0%. The standard (Tokyo) 1980, B, 898. conditions were chosen to be 250 OC, 20 atm, space velocity = (24) Malatesta, L.; Caglio, G.; Angoletta, M. Chem. Commun. 1970,532. (25) Angoletta, M.; Malatesta, L.; Caglio, G.J . Organomet. Chem. 1975, 20 mL (NTP) (g of catalyst.min)-l, and CO/H2 feed molar ratio 94., 99. -= 1. In a blank test with the reactor packed with NaY zeolite, (26) McVicker, G. B.; Baker, R. T. K.; Garten, R. L.; Kugler, E. L. J . no hydrocarbon formation was observed. Catal. 1980, 65, 207. ~
4832 J. Am. Chem. SOC.,Vol. 115, No. 11, 1993
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Figure 3. Infrared spectra characterizing the reactivity of NaY zeolitesupported [h6(c0)16] with edge-bridging ligands in air at 25 OC: (A) NaY zeolite-supported [Ir6(co)16] with edge-bridging ligands; (B)after exposure to air at 25 OC for 5 min; (C) after exposure to air at 25 OC for 15 min; and (D) after subsequent treatment in CO at 125 OC for 30 min.
Typical product distributions with the zeolite-supported Ir catalyst, which was prepared from the adsorbed [Ir(C0)2(acac)] in NaY zeolite, on stream for 1 day and 8 days are shown in parts A and B of Figure 4, respectively. The catalyst gave relatively high yields of (22x4 hydrocarbons, predominantly alkanes. Products with >5 carbon atoms were formed in only very low yields at the low conversions observed (-0.3%). The catalyst showed major changes in product distribution during the first 12 h on stream; most notable was the increased rate of formation of products with more than 3 carbon atoms relative to those with 1 or 2 carbon atoms. After 12 h on stream, the selectivity changed only slightly. The catalysts that had been on stream for 1 day and 8 days were removed from the reactor in the absence of air; they were yellow, with no visible indication of metallic iridium. The hydrocarbon product distributions after 1 day and after 8 days on stream were approximately the same, as shown in the Schulz-Flory plots of Figure 5 , A and B, respectively. Substantial deviations from linear Schulz-Flory plots are evident, with maxima at C3. The good stability of the zeolite-supported catalyst provided an opportunity for investigation of the kinetics of CO hydrogenation. The effects of space velocity, pressure, and feed coInposition on catalyst performance were investigated under steady-state conditions. The conversion to hydrocarbons was observed to be proportional to the inverse space velocity, confirming that the conversions were differential and determined reaction rates directly. High pressures of CO + H2 (about 20 atm) were found to be necessary for maintaining stable catalyst performance. At lower pressures, the catalyst deactivated slowly in operation. After 24 h in operation at 1 atm and otherwise the same conditions stated above, the used catalyst removed from the reactor was dark gray, consistent with the formation of iridium metal particles on the outer zeolite surface. The product distribution observed with the deactivated catalyst was different from those stated above. The
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Time on Stream, h Figure 4. CO hydrogenation catalyzed by zeolite-supported iridium carbonyl clusters: dependence of product distribution on time on stream in a flow reactor up to (A) 1 day and (B) 8 days. Reaction conditions: 250 OC, 20 atm, CO/H2 = 1.0 (molar).
yields of methane and ethane were higher; these product distributions are characteristic of the linear Schulz-Flory plots observed for conventional supported metal catalysts. To determine the effect of high HZpartial pressures on catalyst stability, a sample of [Ir(C0)2(acac)]-containing zeolite was treated for 24 h at 250 OC and 20 atm in flowing CO/H2 with a molar ratio of 1 /3. The activityof thiscatalyst increased during operation in the flow reactor. The activity of the catalyst after 24 h on stream was about 7 times higher than that observed for thecatalyst usedinequimolarCO+ Hz. Whentheformercatalyst sample was removed from the reactor, the upstream end of the catalyst bed was black, consistent with the conclusion that iridium metal particles had formed on the zeolite outer surface after treatment at the relatively high H2 partial pressure. The iridium metal is evidently more active catalytically than the encaged species that had been present in the zeolite. Catalyst Characterizationby Infrared Spectroscopy. A sample of catalyst, which was prepared from the adsorbed [Ir(CO)Z(acac)] in NaY zeolite and had been used in CO hydrogenation with an equimolar CO + H2 feed, was removed from the reactor after 1 day on stream and characterized by infrared spectroscopy. Another sample was used for 8 days and then removed from the reactor and characterized. Both samples catalyzed CO hydrogenation at 20 atm and 250 OC with a feed of equimolar CO + Hz; the two samples exhibited equivalent catalyst performance (Figures 4 and 5). After removal from the reactor, each catalyst sample was yellow (suggesting the presence of iridium carbonyl species); there was no visual indication of metallic species. The infrared spectra of these two used catalysts are shown in Figure 6,A and B, respectively. The spectra of the two samples are virtually the same, and the spectra in the vco region are almost the same as the spectrum of the isomer of [Ir6(co)16] (in THF solution) that has four face-bridging ligands,24J5 except that the frequencies of the terminal CO bands of the encaged cluster are shifted to higher frequencies and that of the bridging ligand is
J. Am. Chem. SOC.,Vol. 115, No. 11, 1993 4833
Zeolite- Entrapped Hexairidium Cluster Catalysts
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Carbon Number, n Figure 5. Hydrocarbon product distributions in CO hydrogenation catalyzed by zeolite-supported iridium carbonyl clusters: (A) 1 day on stream and (B) 8 days on stream.
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Figure 6. Infrared spectra in the YCO stretching region of used CO hydrogenation catalysts after exposure to equimolar CO + H2 at 20 atm and 250 OC for (A) 1 day and (B) 8 days.
shifted to a lower frequency relative to those of the cluster in solution. These results suggest that the predominant form of the iridium in the catalyst samples was the isomer of [Ir6(co)16] with face-bridging ligands, which was stable under reaction conditions. Attempts to extract iridiumcarbonyls from the two usedcatalyst samples with THF or with [PPN] [Cl] in THF wereunsuccessful. The supernatant solutionsremained colorless and had nocarbonyl absorptions, consistent with the stableentrapment of the carbonyl clusters in the zeolite. Catalyst Characterizationby EXAFS Spectroscopy. The two used catalyst samples were also characterized by EXAFS spectroscopy. The normalized EXAFS function for each sample was obtained from the average of the X-ray absorption spectra from two scans by a cubic spline background subtraction. The EXAFS function was normalized by division by the height of the absorption edge. The raw EXAFS data characterizing the catalyst that had been on stream for 1 day (Figure 7A) show oscillations up to a valueofk, thewavevector,ofabnut 14A-1,indicatingthepresence of near-neighbor high-Z backscatterers, which are inferred to be
Ir. Since the infrared data indicate that carbonyl ligands were also present in the sample, the data were first analyzed for Ir-Ir, Ir-C, and I r a * interactions (where O* refers to carbonyl oxygen). The EXAFS analysis was done with experimentally determined reference files that aredescribed in thesection entitled EXAFS Reference Data. The raw EXAFS data characterizing the 1-day sample were Fourier transformed with a kZ weighting over the range 3.52 < k < 14.35 A-l with no phase correction. The Fourier-transformed data were then inverse transformed in the range 0.40 < r < 3.35 A (where r is the distance from the absorber Ir atom) to isolate the major contributions from lowfrequency noise and higher-shell contributions. With the Koningsberger difference file t e c h n i q ~ e , the ~ ~ ~Ir-~ * Ir contribution, the largest in the EXAFS spectrum, was then estimated. Since the I r a * contribution was found to bestrongly coupled with the Ir-Ir contribution, these two contributions had to be analyzed simultaneously. The structural parameters were estimated initially by fitting the data in the high-k range (7.5 < k < 14.0 A-I). The multiple scattering associated with Ir-C-O* groups was found to be significant, with Ir-support and Ir-C contributions being insignificant in this range. Further analysis, following the subtraction of the calculated Ir-Ir and Ir-O* contributions from the raw data, led to characterization of the Ir-Ct and Ir-Cb contributions (where the subscripts t and b refer to terminal and bridging, respectively). The fit of the raw data with the sum of the four contributions was still not satisfactory, and it was inferred that another contribution had to be accounted for. The fifth contribution involved another low-atomic-number backscatterer, which was assumed to be the oxygen of the zeolite, referred to as Ir-Osupport. The structural parameters characterizing the Ir-Ct, Ir-Cb, and Ir-Osupport contributions were determined by fitting the residual spectrum. The initial guesses for parameter estimation were obtained by adjusting the coordination parameters to give the best fit of the residual spectrum in r space. The calculated Ir-C and Ir-Osupportcontributions were then subtracted from the raw data. Better parameters for the Ir-Ir and Ir-O* contributions were then estimatedby fittingthe residual spectrum. The iteration was continued until good overall agreement was obtained. The parameters determined in this fitting routine are summarized in Table I, and the comparisons of the data and the fit, both in k space and in r space, are shown in Figure 7B-D. The residual spectrum determined by subtracting the Ir-Ir IrOsuppat contributionsfrom the EXAFS data (which gives evidence of the carbonyl ligands) is shown in Figure 7E. The number of parameters used to fit the data in this mainshell analysis is 20; the statistically justified number is approximately 21, estimated from the Nyquist theorem,29 n = (2AkArl r ) 1, where Ak and Ar respectively are the k and r ranges used in the forward and inverse Fourier transforms (Ak = 10.83 A-I;
+
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Ar = 2.95 A).
The EXAFS data characterizing the catalyst sample that had been used 8 days in the flow reactor were analyzed in the same way. Thedata arevery similar to thosecharacterizing the sample used for only 1 day (Figure 8). The EXAFS parameters are also very similar, as summarized in Table 11. The data show that about 80% of the iridium was in the form of the carbonyl clusters in this used catalyst sample. Infrared Results Characterizing the Decarbonylation of [Ir6(CO),,] in NaY zeolite. The zeolite incorporating [Ir6(co)16] with edge-bridging ligands was treated in flowing Hz at 1 atm and 100 OC for 30 min. The infrared spectrum was unchanged. (27) Kirlin, P. S.;van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J . Phys. Chem. 1990, 94, 8439. (28) van Zon, J. B. A. D.; Koningsberger, D. C.; van’t Blik, H. F. J.; Sayers, D. E. J . Chem. Phys. 1985,82, 5742. (29) Koningsberger, D. C.; Prins, R. X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and X A N E S Wiley: New York, 1988; p 395.
4834 J. Am. Chem. SOC.,Vol. 115, No. 11. 1993
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Figure 7. Results of EXAFS analysis obtained with the best calculatql coordination parameters characterizing zeolite-supported Ir carbonyl after exposure to equimolar CO Hz at 20 atm and 250 "C for 1 day: (A) raw EXAFS data; (B) experimental EXAFS (solid line) and sum of the calculated Ir-Ct Ir-Cb I r a * Ir-OSupport contributions (dashed line); (C) imaginary part and magnitude of Fourier transform (k' weighted, Ak Ir-Ir = 3.80-13.80 A-1) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-Ct Ir-Cb I r a * + Ir-Osupport contributions (dashed line); (D) imaginary part and magnitude of Fourier transform (k3 weighted, Ak = 3.80 13.80A-I) of experimental EXAFS (solid line) and sum contributions (dashed line); (E) residual spectrum illustrating the contributions of carbonyl of the calculated Ir-Ir Ir-C, Ir-Cb Ir-0* Ir-Osupport Ir-Osupport groups-imaginary part and magnitude of Fourier transform (k3weighted, Ak = 3.80-13.80A-I) of raw data minus calculated Ir-Ir EXAFS (solid line) and calculated Ir-C, Ir-Cb Ir-0* EXAFS (dashed line).
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Table I. EXAFS Results Characterizing the NaY Zeolite-Supported Iridium Carbonyl Species Prepared from [Ir(CO)z(acac)] after Treatment in Equimolar CO + H2 at 250 OC and 20 atm for 1 dayRqb shell N Ir-Ir 3.50 Ir-CO: 1.98 Ir-C, Ir-Cb 1.80 Ir-O* 1.82 Ir-Owppor, 0.76
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AEo,eV 1.81
EXAFSref Pt-Pt
1.89 2.21 3.02 2.13
0.0018 0.0010 0.0036 0.0030
2.42 -2.85 -5.05 -5.26
Ir-C Ir-C Ir-0. Pt-0
Notation: N, coordination number for absorber-backscatterer pair;
R,radial absorber-backscatterer distance; Aa2, Debye-Waller factor (difference with respect to reference compounds); AEo, inner potential correction (correction of the edge position). Estimated precision: N, +20% (Ir-OLyppor,, *30%);R, k2% (Ir-Ir, +1%); ACT*,k30% AEo,+lo%.
As the temperature increased beyond 125 OC, the bands broadened, decreased in intensity, and shifted to lower frequencies. After the sample had been held for 2 h at 300 OC in Hl, the carbonyl bands had disappeared, indicating that the sample had
been decarbonylated. The decarbonylated sample was then evacuated at 300 O C for 30 min and cooled under vacuum to 25 "C,it was beige. The zeoliteincorporating [Ir6(co)16] withfacebridgingligands was treated with Hz in the same way. The sample was stable in H2 at 100 OC for 30 min, but at higher temperatures the carbonyl bands decreased proportionately to each other in intensity, with only a slight change in frequencies. These bands disappeared after the sample had been held for 2 h in H2 at 300 OC. The sample was then evacuated at 300 OC for 30 min and cooled under vacuum to mom temperature; it was beige. EXAFS Results Characterizing the Decrrrbonylated Iridium Clusters. The EXAFS data characterizing the decarbonylated sample formed from theisomer of [Irf,(CO)16]with face-bridging ligands (Figure 9A)were analyzed by a method that is nearly the same as that stated above for the used catalysts. The EXAES data were Fouriertransformedwith kzweightingand nocorrection over the useful range (3.68 < k < 14.15 A-I). The major contributions were isolated by inverse Fourier transformation in therange0.57 < r < 3.32A. TheIr-Ircontributionwasestimated
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Figure 8. Results of EXAFS analysis obtained with the best calculated coordination parameters characterizing zeolite-supported Ir carbonyl after exposure to equimolar CO H2 at 20 atm and 250 OC for 8 days: (A) raw EXAFS data; (B) experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-C1 + Ir-Cb + Ir-O* Ir-OSuppncontributions (dashed line); (C) imaginary part and magnitude of Fourier transform (k' weighted, A& = 3.80-13.80 A-I) of experimental EXAFS (solid line) and sum of the calculated tr-Ir Ir-C1 + Ir-Cb + I r a * Ir-Osupprlcontributions (dashed line); (D) imaginary part and magnitude of Fourier transform (k3weighted, Ak = 3.80-13.80 A-I) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir + Ir-Cl + Ir-Cb Ir-O* + Ir-Osupprlcontributions (dashed line); (E) residual spectrum illustrating the contributions of carbonyl Ir-Osupprl groups-imaginary part and magnitude of Fourier transform (k3weighted, A& = 3.80-13.80 A-') of raw data minus calculated Ir-Ir EXAFS (solid line) and calculated Ir-C, + Ir-Cb I r a * EXAFS (dashed line).
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+
Table 11. EXAFS Results Characterizing the NaY Zeolite-Supported Iridium Carbonyl Species Formed after Treatment of the Sample Prepared from [Ir(CO)z(acac)] in Equimolar CO + H2 at 250 OC and 20 atm for 8 day& shell
N
R,A
Ao2,A2
AEo, eV
EXAFS reference
Ir-Ir Ir-CO: Ir-Cl Ir-Cb Ira* Ir-Osuppr,
3.24
2.77
0.0015
1.85
Pt-Pt
1.71 1.67 1.78 0.86
1.89 2.22 3.01 2.14
0.0029 0.0017 0.0036 0.0038
1.63 -3.39 -5.86 -6.77
Ir-C Ir-C Ira* Pt-O
a Notation as in Table I. Estimated precision: N , *20% (Ir-Osupprl, *30%); R, *2% (Ir-Ir, *l%); Ao2, *30%; AE,, &lo%.
by calculating an EXAFS function that agreed as closely as possible with the experimental results in the high-k range (7.5 < k < 14.0 A-1); the metal-support contributions in this region are small. An EXAFS function calculated with the first-guess parameters was then subtracted from the data, with the residual spectrum being expected to represent the Ir-Os,pPrl interactions. The difference file was estimated with two Ir-0 contributions,
as both and 1ong32933 metal-support oxygen distances have been frequently observed. As a first approximation, only four free parameters were estimated (,ha2, the Debye-Waller factor, and UO, the inner potential correction, were set equal to 0) to shorten the computational time. The first-guess Ir-Ir and Ir-OsupPrtcontributions were then added and compared with the raw data in r space, and the fit was not yet satisfactory. Then the Ir-OsupPrtcontribution was subtracted from the data and better parameters for the Ir-Ir contribution were estimated. The improved fit for the Ir-Ir contribution was subtracted from the data, and more accurate parameters for the contributions of the metal-support interface were determined by fitting the metal-support contributions to (30)Emrich, R.J.; Mansour,A. N.;Sayers, D. E.; McMillan, S.T.; Katzer, J. R. J. Phys. Chem. 1985,89,4261.
(31) Lytle, F.W.; Greegor, R. B.;Marques, E. C.; Via, G. H.; Sinfelt, J. H. J. Catal. 1985,95, 546. (32) Koningsberger, D. C.;van Zon, J. B.A. D.;van't Blik, H.F. J.; Visser, G. J.; Prins, R.; Mansour, A. N.; Sayers, D. E.; Short, D. R.; Katzer, J. R. J. Phys. Chem. 1985,89,4075. (33) Koningsberger, D. C.; Martens, J. H. A.; Prins, R.; Short, D. R.; Sayers, D. E. J . Phys. Chem. 1986,90,3047.
4836 J. Am. Chem. SOC.,Vol. 115, No. 11, 1993
Kawi et al. 5
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Figure 9. Results of EXAFS analysis obtained with the best calculated coordination parameters characterizing zeolite-supported Ir cluster prepared by decarbonylation of the Ir carbonyl cluster in NaY zeolite at 300 OC in H2: (A) raw EXAFS data; (B) experimental EXAFS (solid line) and sum of the calculated Ir-Ir + Ir-C + Ir-Osupprtcontributions (dashed line); (C) imaginary part and magnitude of Fourier transform (k'weighted, Ak = 3.66-13.80k') of experimental EXAFS (solid line) and sum of the calculated Ir-Ir + Ir-C + Ir-Osupprtcontributions (dashed line); (D) imaginary part and magnitude of Fourier transform ( k 3 weighted, Ak = 3.66-13.80 k') of experimental EXAFS (solid line) and sum of the calculated Ir-Ir + Ir-C + Ir-Osuppr, contributions (dashed line); (E) residual spectrum illustrating the EXAFS contributions characterizing the metal-support interaction-imaginary part and magnitude of Fourier transform (k3-weighted, A& = 3.66-10.00 A-I) of raw data minus calculated Ir-Ir + Ir-C EXAFS (solid line) and calculated I r a , + I r a s EXAFS (dashed line).
the residual spectrum with all eight parameters; the initial guesses for parameter estimation were determined by adjusting the coordination parameters to give the best agreement with the residual spectrum, both in k space and in r space. Even after many iterations, the fit was not good in the low-r region. It was thus inferred that another small contribution, attributed to a low-Z backscatterer, had to be accounted for. A difference file was calculated by subtracting the best estimated Ir-Ir + IrOsupport contribution from the experimental EXAFS function. Theadditionalcontributionwascalculated by fitting thedifference file with four adjustable parameters. The additional low-Z scatterer is not identified; it may be carbon remaining from the carbonyl ligands, and we tentatively refer to the contribution as Ir-C. The Ir-Ir, Ir-C, and two Ir-Osupport contributions were then added, representing the overall fit of the data. To show the goodness of the fit for both the high-Z (Ir) and low-Z (0,C) contributions, the raw data are compared with the fit in both k space (with k2 weighting) and in r space (with both k' and k3 weighting) (Figure 9B-D). The agreement is good.
Table 111. EXAFS Results Characterizing the Iridium Clusters Formed by Decarbonylation of [Ir6(co)16] with Face-Bridging co Ligands in NaY Zeolite at 300 OC in Hpb shell
N
Ir-Ir 3.60 Ir-Osupprt: 1.01 Ir-0, 1.85
R,A
Au2, A2
A&, eV
EXAFS reference
2.71
0.0035
-2.00
Pt-Pt
2.17 2.71
0.0070 0.0036
-6.20 -6.11
Pt-o Pt-O
Notation as in Table I; the subscripts s and 1refer to short and long, respectively. Estimated precision: N,f20% (Ir-Ol I r a s , &30%); R, f2% (Ir-Ir, Ala); Au2, f3096, AEo, &lo%.
+
The structure parameters are shown in Table 111, and the IrOsupport contributions are shown in the difference file of Figure 9E. The number of parameters used to fit the data in this mainshell analysis is 16; the statistically justified number, calculated as above, is approximately 19. Recarbonylationof the DecarbonylatedIridium Clusters. The decarbonylated sample formed from the [Ir6(co)16] isomer with face-bridging ligands (the used catalyst, Figure 10A) was exposed
J. Am. Chem. SOC.,Vol. 115, No. 1 1 , 1993 4837
Zeolite-Entrapped Hexairidium Cluster Catalysts
I
IO
2100
I
I
I
2000 1900 1800 WAVENUMBER, cm-1
1
Figure 10. Infrared spectra characterizing the recarbonylation of the decarbonylated iridium cluster formed from treatment of [Ir6(C0)16] with face-bridging ligands in NaY zeolite: (A) decarbonylated iridium cluster in NaY zeolite; (B) after treatment in CO at 125 OC for 2 h; (C) after treatment in CO at 125 OC for 6 h; (D) after treatment in CO at 175 O C for 2 h; and (E) after treatment in CO at 175 OC for 5 h.
to CO in the infrared cell as the temperature was raised. When the temperature reached 125 OC, infrared bands in the carbonyl region grew in at 2082,2040,and 1815 cm-1 (Figure 10B).The growth in intensity continued for about 6 h, with the spectrum finally becoming virtually the same as that of the [Ir6(co)16] isomer with edge-bridging ligands (Figure 1OC). Again, the sample was light yellow. When the sample was recarbonylated at temperatures >175 OC, a terminal band at 2098 cm-I and a bridging carbonyl band at 1730 cm-1 appeared in the infrared spectrum, which is attributed to the [Ir6(co)16] cluster with facebridging ligands (Figure 10D). After about 5 h at 175 OC, the spectrum became virtually the same as that of the [Ir6(co)16] cluster with face-bridging ligands (Figure 10E). These spectra show that the carbonylation-decarbonylation process was reversible; it was repeated three times with no noticable change in the infrared spectra of the carbonylated forms. When the decarbonylated cluster formed from the isomer of [Ir6(CO)161 with edge-bridging ligands was recarbonylated, the results were the same as those stated in the preceding two paragraphs. EXAFS Results Characterizing Recarbonylated Indium Carbonyl Clusters. The sample that had been formed by decarbonylation and subsequentrecarbonylationof the zeolite incorporating the [Ir6(Co)161 with face-bridgingligandswas also characterized by EXAFS spectroscopy. The data are nearly the same as those characterizing the sample incorporating the original cluster with face-bridging ligands. The method of data analysis is virtually the same also. The results are shown in Figure 1 1 and summarized in Table IV. The results are almost identical to those presented for the sample prior to decarbonylation. These results confirm the conclusion from infrared spectroscopy that the decarbonylation-recarbonylation process was reversible. Discussion Evidence of [Ir6(CO)161in Zeolite Supercages: Parallelsbetween Iridium Carbonyl Chemistry in Solution and in Zeolite Cages.
[Ir4(CO)12]is synthesized in solution by carbonylationof aqueous iridium salts3"36 or bycarbonylation of [Ir(CO)2(acac)]in undried THF.37 Thus we suggest that water played a role in the synthesis of [Ir4(CO)12]. In basic solutions, the reductive carbonylation of [Ir4(CO)12]with KOH in methanol under CO yields a family of iridium carbonyl cluster anions with nuclearities of 4,6, and 8 ; 2 5 these clusters incorporate both terminal and bridging CO ligands. The reaction initially gives [HIr4(CO)II]-38 and then [I~s(CO)ZZ]~39 and [Ir6(C0)15]2-.40.41 The latter is the most stable of these anionic species formed in the carbonylation of iridium precursors in basic ~olutions.25~~~ In acidic media, [Ir6(C0)15]2-isconverted to [Ir6(co)16]; both isomers mentioned above have been synthesized in this way.22J5,41 The basic surface of MgO is also an efficient medium for the synthesis of anionic metal carbonyl clusters.424s This surface has also been used as a medium for synthesis, in the absence of solvents, of [Os& (CO) I 41 2-,46 [Os OC(CO) 24]2-,46,47 [H Ir4(C0)11]-,4~,~~ [ I r ~ ( C 0 ) 2 ~ ] ~and - , 4[Ir6(CO)15]2-;43,45 ~.~~ theanions could be extracted from the surface by cation metathesis. The chemistry of osmium and iridium carbonyl clusters on the surface of MgO closely parallels the chemistry of these metal carbonyl clusters in basic solutions. Someof these anionic carbonyl clusters have been shown to be stabilized on the MgO surface during catalytic CO h y d r o g e n a t i ~ n . ~ ~ . ~ ~ Similarly, it has also been shown that the supercages of NaX zeolites are sufficiently basic to provide an efficient medium for the synthesis of anionic iridium carbonyl clusters.6 These cages have been used as a medium for synthesis of [HIr4(CO)II]- and [Ir6(CO)15]2-in good yields. The chemistry of iridium carbonyl clusters in the cages of NaX zeolite parallels the chemistry of the metal carbonyl clusters in basic solutions and on the basic MgO support. The [Ir6(C0)15]2-cluster in NaX zeolite has also been shown to be maintained during catalytic CO hydrogenation.6J6 On the weakly basic y-A1203surface, however, the chemistry of metal carbonyls is different, being similar to that occurring in nonbasic solutions. For example, iridium precursors react on the y-A1203surface in the presence of CO to give [Ir4(C0)12].37The formation of neutral iridium carbonyl clusters reported here in the NaY zeolite is evidently similar to that occurring on the y-A1203surface. The result is not surprisingbecause NaY zeolite is less basic than NaX zeolite (the Si/Al atomic ratio of the NaX was about 2.5and that of the NaY was about 5.0). These results give an indication of the possibilities for modification of the reactivity in the solvent-likecages of the zeolites by changing the Si to A1 ratio. Since [Ir4(CO)12]and the two isomers of [Ir6(co)16] are slightly soluble in THF, the lack of extraction of these clusters from NaY zeolite with THF indicates that the clusters were trapped in the zeolite cages. The results are consistent with a (34) Malatesta, L.; Caglio, G.; Angoletta, M. Inorg. Synth. 1972, 13, 95. (35) Della Pergola, R.; Garlaschelli, L.; Martinengo, S. J . Orgunomet. Chem. 1987, 331, 271. (36) Pruchnik, F. P.; Wajda-Hermanowicz, K.; Koralewicz, M. J . Organomef. Chem. 1990, 384, 381. (37) Kawi, S.; Chang, J.-R.; Gates, B. C. J . Phys. Chem., in press. (38) Bau, R.; Chiang, M. Y.; Wei, K.; Garlaschelli, K.; Martinengo, S.; Koet.de, T. F. Inorg. Chem. 1984, 23, 4758. (39) Demartin, F.; Manassero, M.; Garlaschelli, L.; Raimondi, C.; Martinengo, S.; Canziani, F. J . Chem. SOC.,Chem. Commun. 1981, 528. (40) Demartin, F.; Manassero, M.; Sansoni, M.; Garlascheni, L.; Martinengo, S. J. Chem. SOC.,Chem. Commun. 1980,903. (41) Stevens, R. E.; Lin, P. C. C.; Gladfelter, W. L. J. Organomet. Chem. 19115 - - - -, -287 . , -133 --. (42) Maloney, S. D.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. Cutal. Lett. 1990, 5, 161. (43) Maloney, S. D.; Kelley, M. J.; Koningsberger, D. C.; Gates, B. C. J . Phys. Chem. 1991, 95, 9406. (44) Maloney, S. D.; Kelley, M. J.; Gates, B. C. J . Organomet. Chem. 1992, 435, 377. (45) Kawi, S.; Gates, B. C. Inorg. Chem. 1992, 31, 2939. (46) Lamb, H. H.; Fung, A. S.;Tooley, P. A,; Puga, J.; Krause, T.R.; Kelley, M. J.; Gates, B. C. J . Am. Chem. SOC.1989, 111, 8367. (47) Lamb, H. H.; Gates, B. C. J. Am. Chem. Soc. 1986, 108, 81. (48) Kawi, S.; Gates, B. C., to be published.
Kawi et al.
4838 J. Am. Chem. SOC.,Vol. 115, No. 11, 1993 I
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r , 8, Figure 11. Resultsof EXAFS analysis obtained with the best calculated coordination parameters characterizing the recarbonylation of the dccarbonylated iridium cluster formed from treatment of [Ir6(co)16] with face-bridging ligands in NaY zeolite: (A) raw EXAFS data; (e) experimental EXAFS (solid line) and sum of the calculated Ir-Ir + 1r-G + Ir-Cb + I r a * + Ir-O,,pport contributions (dashed line); (C) imaginary part and magnitude of Fourier transform (&I weighted, A& = 3.80-13.80A-I) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir + Ir-Ct + Ir-Cb + 1r-O. + Ir-Oaupporlcontributions (dashed line); (D) imaginary part and magnitude of Fourier transform (&3 weighted, Ak = 3.80-13.80A-I) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-Ct + Ir-Cb Ir-O* Ir-Osupprlcontributions (dashed line); (E) residual spectrum illustrating the contributions of carbonyl groups-imaginary part and magnitude of Fourier transform (k3weighted, A& = 3.80-13.80 A-I) of raw data minus calculated Ir-Ir + Ir-OsupprtEXAFS (solid line) and calculated Ir-Ct + Ir-Cb + I r a * EXAFS (dashed line).
+
+
Table IV. EXAFS Results Characterizing the Species Formed by CO Treatment of Decarbonylated Iridium Clusters in NaY ZeoliteoSb shell N R.A Au2. A2 AEo. eV EXAFS reference Ir-Ir 3.40 2.76 0.0013 -0.80 Pt-Pt Ir-CO: Ir-Cl 1.99 1.85 0.0023 4.70 Ir-C Ir-Cb 1.60 2.24 0.0022 -5.46 Ir-C Ira* 1.69 2.99 0.0027 -5.80 Ir-O* Ir-OJupprtC 0.82 2.10 0.0045 -8.00 Pt-O ~
~~~~
~~~
a Notation as in Table I. Estimated precision: N,*20% (Ir-Osupprt, f30%); R,f2% (Ir-Ir, &I%); Au2, *30%; AEo, *lo%.
shipin-a-bottlesynthesis in the zeolitesupercages. The precursor [Ir(CO)2(acac)]issmall enough to fit through thezeoliteapertures and diffuse into the interior of the zeolite crystallites. Even larger metal carbonyls have been shown to fit: [CpM(CO)z], [Cp*M(CO)z], and [CpM(CzH4)2][M = Rh, Ir, Cp = C5H5; Cp* = (CH3)5C5].49[Ir4(C0)I~] (with a diameter of about 9 A) and the two isomers of [Ir6(co)16] (with diameters of about 11 A) (49) Ozin, G.A.; Haddleton, D.
6710.
M.;Gil, C. J. J. Phys. Chem. 1989, 93,
+
are small enough to fit in the supercages of zeolite Y (which have diameters of about 12 A) but too large to fit in the sodalite cages and too large to diffuse rapidly through the apertures (which have diameters of about 7.4 A). Thus it is concluded that the clusters were trapped in the supercages and not removed when the sample came in contact with THF. C O I l f k " t i O l l Of StrUCtUre Of Z d h - E n h p p e d [ k s ( c 0 ) , 6 ] by EXAFS Spectroscopy. Both the infrared and EXAFS results are consistent with the inference that the iridium carbonyl species formed in high yield from [Ir(C0)2(acac)] in NaY zeolite after treatment in equimolar CO Hz at 20 atm and 250 OC for 1 day was the isomer of [If6(Co)I6] with face-bridging CO ligands. The evidence for this cluster is the following: (1) the close agreement between the infrared spectrum of the zeolite-supported iridium carbonyl and that of the THF solution of the isomer of [Ir6(co)16] with face-bridging CO ligands (however, infrared spectra are usually not sufficient to identify metal carbonyls in zeolitesz1)and (2) the EXAFS spectrum of the zeolite-entrapped iridium carbonyl. The EXAFS results provide the strongest evidence, showing that the backscatterersin theimmediatevicinity of the Ir absorber
+
Zeolite- Entrapped Hexairidium Cluster Catalysts atoms in the zeolite-supportediridium carbonyl included Ir and low-Z backscatterers identified by the multiple scattering effect as C and 0.. We infer, as follows, that the relative contributions of these backscattering atoms support the identification of the zeolite-entrapped species as [Ir6(CO)l6]: All the Ir atoms in crystalline [Ir6(co)16] with face-bridging ligands are stereochemically equivalent, with each bonded to four Ir atoms at an average distance of 2.779 A, to two terminal carbonyl ligands with an average Ir-Ct distance of 1.886 A, and to two bridging carbonyl ligands with an average Ir-Cb distance of 2.20 A.22The EXAFS results characterizing the supported iridium carbonyl (Table I) match, within experimental error, the crystallographic data for [Ir6(CO)161 with face-bridging ligands. They indicate an Ir-Ir coordination number of 3.5 (which is equal to 4, within the experimental error of 20%) with an average Ir-Ir distance of 2.77 A, an Ir-C, coordination number of 2.0 with an average distance of 1.89A, and an Ir-Cb coordination number of 1.8 with an average distance of 2.21 A. Although the structural data characterizing the Ir-Ir, Ir-C,, and Ir-Cb interactions agree well with the crystallographic data, the results characterizing the Ir-O* interaction (Table I) do not. The discrepancy between the sum of the Ir-C, (2.0) and Ir-Cb (1 -8) coordination numbers, on the one hand, and the I r a * coordination number (1.8), on the other, can be understood on the basis of multiplescatteringe f f e c t ~ ,as 4 ~follows: ~ ~ ~ The positions of the oxygens of the terminal and bridging carbonyl ligands in thecrystalline [h6(c0)16] with face-bridgingligands areknown.22 The crystallographic distance between the Ir and 0 atoms of the terminal carbonyl ligands is virtually the same as the crystallographicdistance between the Ir and 0 atoms of bridging carbonyl ligands. However, the Ir-C-Ot angle is 177O, whereas the Irc a b angle is 133°.22 When the 1r-C-0 bond angle is greater than approximately 140°, the EXAFS phase shift and amplitude function depend strongly on the positions of the atoms because of multiple scatteringeffects.sO In contrast, the multiple scattering effect is not expected to be significant for bridging carbonyl oxygens in [Ir6(co)16], since the Ir-C-Ob angle is only 133O. Since the terminal and bridging carbonyl oxygen atoms are located at nearly the same distance from Ir, it is difficult to separate the two contributions in the EXAFS analysis and to determine an accurate estimate of the overall I r a * coordination number. Consequently, we infer that the discrepancy between the sum of the Ir-Ct and Ir-Cb coordination numbers and the Ir-O* coordination numbers determined by the EXAFS data is a result of there being two types of carbonyl oxygen contributions at nearly the same distance but experiencing different degrees of multiple scattering that lead to different phase shifts, which may give a negative interference and a low estimate of the I r a * coordination number. Thus the EXAFS data are consistent with the infrared data and the conclusion that [Ir6(co)16] with face-bridging ligands was present in the zeolite. The EXAFS data are not consistent with the presence of significant amounts of the isomer of [Ir6(co)16] with edge-bridging CO ligands, which would be characterized by an Ir-Cb coordination number of 1.33 in contrast to the observed value of 1.8. There was another contribution evident in the EXAFS data, indicating the presence of a small amount of an Ir-containing species in addition to [Ir6(C0)16]. The additional contribution was needed for a good fit of the EXAFS data. It may arise from unconverted [Ir(CO)z(acac)] or other mononuclearspecies.22,sl-53 The results indicate that more than 85% of the Ir atoms were present in the [Ir6(CO)l6] clusters. (50) van Zon, F. B. M.; Kirlin, P.S.; Gates, B. C.; Koningsberger, D. C. J . Phys. Chem. 1989, 93, 2218. (51) Rao, L.-F.;Fukuoka, A.; Kosugi, N.; Kuroda, H.; Ichikawa, M. J. Phys. Chem. 1990, 94, 5317. ( 5 2 ) Yates, J. T., Jr.; Duncan, T. M.; Vaughan, R. M.J. Chem. Phys. 1979, 71, 3908. ( 5 3 ) Wang, H. P.; Yates, J. T., Jr. J. Card. 1984, 89, 79.
J. Am. Chem. SOC.,Vol. 115, No. 11, 1993 4839 Scheme I. Chemistry of Iridium Clusters in NaY Zeolitea N8Y IIr(CO)2(us)l
[Ir,(CO),,] (face) / NaY
[Ir,(CO),, 1 I Nay
nr6]INaY [Ir(CO),]I NaY The terms edge and face refer to the isomers with edge- and facebridging carbonyl ligands, respectively.
In summary, there is good agreement between the EXAFS results characterizing the sample formed from [Ir(CO)~(acac)] in NaY zeolite after treatment in CO H2 at 250 OC and 20 atm for 1 day (Table I) and the crystal structure of the isomer of [Ir6(co)16] with face-bridging ligands.22 We conclude, therefore, that the predominant entrapped species in NaY zeolite after treatment in high-pressure CO + Hz was [Ir6(CO)l6] with face-bridging CO ligands. The intrazeolite chemistry is summarized in Scheme I. Otherss4 have also reported the formation of [Ir6(co)16] in NaY zeolite from a mononuclear iridium carbonyl in the presence of CO. However, the conclusions were based only on infrared spectra, and the authors did not distinguish which isomers of the cluster were present. interactionsof Iridium CarbonylClusterswith the NaY Zeolite Framework. The isomer of [Ir6,(co)l6] with face-bridging CO ligands in NaY zeolite is characterized by principal YCO bands at 2098 s, 2066 w, and 1730 cm-I. The 2098- and 2066-cm-I bands are assigned to the symmetric and asymmetric stretching of the two CO ligands bound to each Ir atom in [Ir6(CO),6]. The 1730-cm-1 band correspondsto the four face-bridging CO ligands, each bound to three Ir atoms. The stretching frequency of the terminal CO ligand is shifted to higher energy by about 20 cm-I and that of the face-bridgingCO ligand is shifted to lower energy by about 35 cm-I than those of the same isomer of [Ir6(co)l6] in THF solution. Similar trends in the infrared band shifts have been observed for the isomer of [Ir6(co)16] that has edge-bridging CO ligands in NaY zeolite, which is characterized by principal carbonyl infrared bands at 2082 s, 2040 m, and 1816 m cm-I. The 2082and 2040-cm-I bands are assigned to the symmetric and asymmetric stretching of the two terminal CO ligands bound to each Ir atom in [Ir6(co)16]. The 1816-cm-1band corresponds to the four edge-bridging CO ligands, each bound to two Ir atoms. The stretching frequency of the terminal CO bond is shifted to higher energy by approximately 5 cm-l and that of the edgebridging CO ligand is shifted to lower energy by approximately 1Ocm-1than thoseofthesameisomer [Ir6(CO)l6] inTHFsolution. These shifts of the terminal and bridging carbonyl bands observed with both isomers of [Ir6(co)16] in NaY zeolite are similar to the shifts observed for metal carbonyls in solutions containing Lewis acids such as Al(CzH&, which have been examined exten~ively.5~*~6 A general pattern appears to be the following: Interaction of a metal carbonyl with a Lewis acid via
+
(54) Gelin, P.;Lefebvre, F.; Elleuch, B.; Naccache, C.; Ben Taarit, Y. ACS Symp. Ser. 1985, 218,469. ( 5 5 ) Shriver, D. F. J. Orgunomet. Chem. 1975, 94, 259. (56) Horwitz, C. P.;Shriver, D. F. Adu. Orgunomer. Chem. 1984,23,219.
4040 J. Am. Chem. SOC.,Vol. 115, No. 11. 1993
Kawi et al.
the oxygen of a carbonyl group typically results in a large decrease Metal carbonyl compounds are well-known examples of of the infrared absorption frequency of the carbonyl group, with fluxional molecules,6’ and some adopt different isomer forms in the stretching frequencies of the noninteracting carbonyl ligands solution. The data characterizing the reductive carbonylation of all shifting to slightly higher frequencies. For example, Shriver iridium carbonyls in the zeolite cages show that the isomer of and c o - w o r k e r ~ 5reported ~ ~ ~ ~ shifts of terminal and bridging [Ir6(co)16] with edge-bridging ligands was formed at a lower carbonyl bands resulting from adduct formation between comtemperature than the isomer of [Ir6(CO)l6] with face-bridging ligands. The observation suggests that the edge-bridging species pounds such as [(C5H~)Fe(C0)2]2and Lewis acids such as Almay be an intermediate in the conversion of the mononuclear (C2H5)3, which led to shifts of 100-300 cm-l to lower energy of iridium carbonyl to the isomer of [h6(C0)16] with face-bridging the bridging carbonyl bands and shifts of 30-70 cm-1 to higher carbonyl groups. energy of the terminal carbonyl bands. Similar shifts have been Although isomers of [Ir6(C0)16],22[CO~(CO)S],~~ and [H3observed for [Rh6(CO)16]in NaY zeolite and attributed to the Rud(C0) 12]-63 have been crystallographicallycharacterized, the interactions of the oxygen atoms of the face-bridging carbonyl present work represents the first evidence of two different isomers ligands with the Lewis acid sites ( A P ions).51 of a metal carbonyl cluster in a cage. The conversion of one of The basicity of the oxygen in carbonyl ligands of metal clusters the isomers of [Ir6(CO)l6] to the other (and the interconversion depends on the CO coordination geometry;57triply bridging of [Ir6(co)16] and [Ir4(CO)12])in NaY zeolite cages suggests carbonyl ligands are more basic than doubly bridging carbonyl that the zeolite cages have a solvent-like character. They may ligands, which are significantly more basic than terminal carbonyl be advantageous media for synthesis of new structures: When ligands. This pattern is borne out in the present results; the the isomers of [Ir6(CO)16]were synthesized in solution, only 10% relatively large shifts in the stretching frequency of the faceyields of each were 0bserved;22.2~a 60% yield was obtained with bridging ligands in [h6(co),6] compared with the edge-bridging a tedious method.41 In contrast, the isomer of this cluster with carbonyl ligands in this cluster in NaY zeolite indicate a stronger facebridging ligands was synthesizedin approximately 85%yield interaction of Lewis acid sites with the oxygen of face-bridging in thezeolitecages. Zeolitecagesappear to facilitate thesynthesis CO ligands than with the oxygen of edge-bridging CO ligands. of palladium carbonyl clusters that are not known in the soluble Such an interaction is expected to result in a net withdrawal of or crystalline~tates.~.~~ However, thereis noknown way toremove electrons from the cluster, decreasing the back bonding to the the clusters intact from the cages. terminal carbonyl ligands, strengthening the C-O bonds, and EvidenceThatIridiumCarbonylsWereTrappedioZeoliteCages shifting the vco bands of the terminal carbonyl ligands to higher and Not hesent Outside Zeolite Crystallites. One of the main frequencies. [Ir4(CO)~2]in the zeolite, which has only terminal issues complicating the synthesis and characterization of metal carbonyl ligands, is not characterized by significant shifts in the clusters in zeolites is the simultaneousformation of metal clusters C-O stretching frequencies, consistent with the weak basicity of or crystallites outside the zeolite crystallites, which is apparently the ligands (Figure 1B). often unavoidable. Much of the reported work with metal clusters In summary, the interactions of the carbonyl ligands of iridium and other nanostructures in zeolites has failed to include evidence carbonyl clusterswith the Lewis acid sites in zeolitesare indicated to establish whether all the clusters were actually confined in the by shifts in the stretching frequencies of the CO ligands, and the cages. pattern parallels that observed for metal carbonyl clusters in The results of the work summarized here are consistent with solutions containing Lewis acids. the inference that virtually all the iridium clusters were formed Reactivity of Indium Carbonyl Clusters in NaY Zeolite Cages. in and confined to the zeolite cages. The evidence is as follows: When the zeolite-supported [Ir6(CO) 161 with edge-bridging (1) The synthesis conditions were chosen to remove iridium ligands was treated with oxygen, a reaction occurred to produce precursors and clusters from the outside of the zeolite crystals. a species characterized by two sharp carbonyl bands, at 2083 and The use of the neutral precursor [Ir(CO)z(acac)] has the 2000 cm-I. The new iridium carbonyl species is assigned as an advantage of allowing removal of unconverted precursor that IrI(C0)z complex on the basis of the comparison of its infrared remainedoutside thecrystallitesby thorough washing with hexane; spectrum with, for example, the spectrum of [Ir(CO)2(acac)] the infrared spectra indicate that the washing did not remove all and that of [Rh(C0)2C1]2.58 We infer that there wasan oxidative the [Ir(CO)2(acac)] from the sample. Hence, we infer that the fragmentation of iridium carbonyl clusters to form iridium precursor remaining after the washing was inside the zeolite cages. subcarbonyls in the zeolite. Such reactions on the surfaces of Thus we infer that these precursors were the source of the iridium amorphous metal oxides have been investigated extensively and carbonyl clusters, which were entrapped in the zeolite cages. used to synthesize nearly uniform, isolated surface-bound en(2) The lack of success in extraction of the iridium carbonyl sembles having the nuclearity of the cluster p r e c u r s ~ r . ~ ~ . ~ ~ . ~ ~ T clusters h e r e from the samples is consistent with the entrapment of the is a parallel between the oxidative fragmentation of supported clusters, consistent with the fact that the clusters are too large metal carbonyl clusters and the disruption of small supported to fit through the zeolite apertures. In contrast, iridium carbonyl aggregates of metal resulting from exposure to CO.57 clusters could be extracted readily from surfaces of metal oxide supports (Mg045and y - k U 2 0 ~ ~that ~ ) have pores large enough The oxidized Ir carbonyl species in NaY zeolite could be to allow the rapid diffusion of the clusters. reductively carbonylated by treatment in CO to give the isomer (3) The evidence of interactions of both isomers of [Ir6(co)16] of [Ir6(CO)16] with edge-bridging ligands, and the cycle could with cations of the support (evidence from the infrared spectra) be repeated many times. The repeatability of the oxidativeis consistent with interactions of the CO ligands with Na+ ions, fragmentation/reductive-carbonylation cycle suggests that the which are exchangeable ions located in the zeolite cages. environment of the zeolite cages played a role in preventing the (4) The reversibility of the cycles of oxidative fragmentation mononuclear iridium carbonyl species from migrating out of the and reductive carbonylation of the iridium carbonyls might not zeolite pores to the outer zeolite surface, thereby hindering the be expected to occur if the iridium species were not in uniform formation of larger iridium clusters and even crystallites. environments such as those provided by the zeolite cages, which we infer help to stabilize the fragmented species and prevent (57) Lamb, H. H.; Gates, B. C.; KnBzinger, H. Angew. Chem., Int. Ed. E n d . 1988. 27. 1127. their agglomerationto form particlesof iridium metal. In Contrast,
?58).Smith,’A. K.; Hugues, F.; Theolier, A.; Basset, J. M.; Ugo, R.; Zanderighi, G. M.; Bilhou, J. L.; Bilhou-Bougnol, V.; Graydon, W. F. Inorg. Chem. 1979, 18, 3104. (59) Gates, B. C. In Catalyst Design: Progress and Perspectives;Hegedus, L. L., Ed.; Wiley: New York, 1987; p 71. (60) Gates, B. C.; Lamb, H. H. 3. Mol. Catal. 1989, 52, 1.
(61) Cotton, F. A. 3. Organomet. Chem. 1975, 100,29. (62) Lichtenberger, D. L.; Brown, T. L. Inorg. Chem. 1978, 17, 1381. (63) Jackson, P. F.; Johnson, B. F. G.; Lewis, J.; McPartlin. M.; Nelson, W. J. H. 3. Chem. Soc., Chem. Commun. 1978,920. (64) Zhang, Z.; Sachtler, W. M. H. 3. Mol. Catal. 1991, 67, 349.
J . Am. Chem. SOC.,Vol. 115, NO.11, 1993 4841
Zeolite-Entrapped Hexairidium Cluster Catalysts [Ir4(CO)121 formed on y-A120337,65 and Si02165366for example, could not be reversibly fragmented and reformed; instead, Ir particles formed. Catalytic Hydrogenation of CO. The NaY zeolite-supported Ir catalyst is characterized by an unusual selectivity in CO hydrogenation; it gave a non-Schulz-Flory distribution of hydrocarbon products (with high yields of propane), which contrasts with the Schulz-Flory distribution that is characteristic of alumina-supported iridium crystallites and virtually all conventional supported metal catalysts. Similar non-Schulz-Flory distributions have been observed with NaY zeolite-supported osmium carbonylI3J4and rhodium carbonyl15catalysts as well as NaX zeolite-supported iridium carbonyl catalysts.16 The predominant form of the metal in each of these zeolite-supported catalysts was suggested to be a metal carbonyl cluster. These results are consistent with the suggestion that the catalytically active species were metal carbonyl clusters entrapped in the zeolite cages, which are evidentlycatalyticallydistinct from conventional supported metals. However, the results do not exclude the possibility of catalysis by undetected small Ir crystallites present with the iridium carbonyl clusters in the catalyst. Since the activity of thecatalyst is low and the activity of Ir metal is relatively high, the observed activity could be explained by only a small amount of Ir metal. The stabilityof the zeolite-supported catalystsmay be attributed to the stabilization of the metal clusters associated with (1) the strength of the metal-metal bonds, (2) the CO ligands provided by the gas-phase reactant, and (3) the geometry of the zeolite pores, with relatively large cages (diameter = 12 A) and small apertures (diameter = 7.4 A) which entrap the clusters and hinder their sintering into unselective metal particles.I3J4 Pressure was found to be crucial for stable performance of these zeolite-supported catalysts. The results characterizing the iridium-containing catalyst reported here show that a pressure of 20 atm of equimolar CO H2 was sufficient to maintain the catalytic performance for at least 1 day. The contrasting results obtained at low pressure indicate that the high pressure of CO is needed to maintain the carbonyl clusters during catalytic CO hydrogenation and thus maintain the catalytic performance. The infrared results obtained at 1 atm of CO + H2 show that the hexairidium carbonyl clusters could not be maintained at temperatures >240 OC; they agglomerated to form metal particles. The catalyst that had been used with a feed containing a 113 molar CO:H2 ratio at 20 atm was black at the upstream end of the catalyst bed, suggesting that Ir metal particles had formed on the zeolite outer surface. We infer that under the relatively high H2 partial pressure and low CO partial pressure, the zeolitesupported iridium carbonyl species fragmented and migrated out through the apertures of the ze01ite.l~The product distribution data characterizing the catalytic reaction are typical of a conventional supported metal catalyst, giving the Schulz-Flory distributionof hydrocarbon products. These results are consistent with the inference that the catalytically active species in the more selective catalysts (those not exposed to low CO/H2 ratios) were trapped in the zeolite cages. Stability of [Ir6(CO)16] during Catalytic CO Hydrogenation. As shown in preceding sections, it is apparent that the catalyst after being used in equimolar CO H2 at 20 atm for 1 day contained predominantly [Ir6(CO)161 with face-bridgingligands. The infrared results characterizing the catalyst that had been used for 8 days were indistinguishablefrom those characterizing the catalyst used for only 1 day. The EXAFS results characterizing the two used catalysts (Tables I and 11) indicate, within experimental error, the same amount of [Ir6(co)l6]; about 80% of the iridium was in the form of these clustersin the used catalysts.
+
+
The maintenance of the activity and selectivity of the catalyst and the lack of change in the amount of [Ir6(CO) 161 are consistent with the suggestion that the isomer of [h6(co)16] with facebridging ligands might be the catalyst precursor; however, there are not sufficient data for identification of the catalytically active species. Decarbonylationof [h6(C0)16]Entrapped in NaY Zeolite. The decarbonylation of [hb(c0)16] in NaY zeolite was carried out in flowing H2 by gradually increasing the temperature. The terminal absorption bands in the infrared spectrum decreased in intensity, broadened, and shifted to lower frequency as the temperature increased. This result is similar to those of Handy et al.,67 who observed that both the terminal and the bridging bands shifted to lower energy as the degree of decarbonylation of [Pt15(CO)30]2-(present as the PPN salt) on y-A1203increased with the sample in flowing He. The decrease of CO coverage leads to a diminished dipole-dipole coupling between adjacent CO molecules and a lower energy shift of the CO absorption bands.68 To investigate the structure ofthe clusters resulting from the decarbonylation, the sample was characterized by EXAFS spectroscopy. The Ir-Ir coordination number characteristic of the sample prepared by decarbonylation of [Ir6(co)16] in NaY zeolite (Table 111), 3.6, is the same, within experimental error, as the crystallographicallydetermined Ir-Ir coordinationnumber of [Ir6(CO)l6] (4). The result suggests that the structure of the Ir cluster frame after decarbonylation resembles that of the octahedral frame of [ITg(C0)16]. The lack of EXAFS evidence of the remaining (higher-shell) Ir atom in the octahedron is inferred to be a consequence of the lack of sensitivityof the EXAFS technique.43 The lack of significant peaks in the Fourier transforms corresponding to higher-shell Ir-Ir neighbors is also consistentwith the inference that there was no significant sintering of the Ir to form crystallites on the outer zeolite surface. There is an analogy between the chemistryof iridium carbonyls in NaY zeolite and that of rhodium carbonyls in NaY zeolite.5' However, the analogy has not been shown to extend to the simple decarbonylation of the clusters; the present results indicate a decarbonylation of the iridium carbonyl without loss of cluster nuclearity, but thedecarbonylationof [Rh6(C0)16]led togrowth in cluster nuclearity, as indicated by EXAFS results.5' The Ir-Ir distance characterizing the decarbonylated Ir6 clusters in NaY zeolite is 2.71 A, which is the same as the Ir-Ir distance in bulk Ir meta1.69 The comparison is not as simple as one would wish, however, as the samples were characterized by EXAFS spectroscopyin the presenceof Hz. Tzou et reported that the Rh-Rh distance of reduced Rh clusters in NaY zeolite was about 2% shorter than that in Rh foil, but their sample had been purged with He. We expect that our reduced samples incorporated adsorbed hydrogen in the EXAFS experiment, and adsorbed hydrogen has been shown to increase the Rh-Rh distance in y-Al203-supported Rh71 and Y zeolite-supported Pt.72 In summary, these observations indicate that the structure of the Ir cluster frame after decarbonylation under mild conditions resembled that of the octahedral frame of [Ir6(CO)16]. The decarbonylated hexairidium clusters are inferred to be similar to those that have been inferred to form by decarbonylation of [Ir6(C0)15]2-supportedon MgO;43it is possible that the clusters are stabilized by the rigid environment of the zeolite cages and migrate and sinter less rapidly than they would on a support with larger pores. (67)Handy, B. E.;Dumesic, J. A.; Langer, S.H. J. Catal. 1990,126,73. (68)Primet, M.J . Catal. 1984,88,273. (69)Wyckoff, R.W. G.Crysral Structures, 2nd ed.; Wiley: New York, 1963 --. Vol. 1. D 10. -(70)Tzou, M.S.;Teo, B. K.; Sachtler, W. M.H. Langmuir 1986,t.773. (71) van't Blik, H. F. J.; van zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . Am. Chem. SOC.1985,107, 3139. (72) Moraweck, B.; Clugnet, G.; Renouprez, A. Surf. Sci. 1979.81,L631. ~
(65)Tanaka, K.;Watters, K. L.; Howe, R. F. J. Catal. 1982,75, 23. (66)Psaro, R.; Dossi, C.; Fusi, A.; Della Pergola, R.; Garlaschelli, L.; Roberto, D.; Sordelli, L.; Ugo,R.; Zanoni, R. J. Chem. Soc., Faraday Trans. 1992,88,369.
Kawi et al.
4842 J. Am. Chem. Sa..Vol. 115, No. 11, 1993
Recarbonylation of tbe Decarbonylated Iridium Clusters. The results show that both isomers of [ I T ~ ( C O )in ~ ~NaY ] zeolite could be decarbonylated; recarbonylation initially gave theisomer of [Ir6(CO)16]with edge-bridging ligands. This could be converted to the isomer with face-bridging ligands, so that the decarbonylation-carbonylation processes were reversible. In contrast, it has typically been found that CO adsorption leads to significant morphological changes in supported clusters of group 8 metals; the processes are usually irreversible. The details of the surface chemistry depend on the metal cluster size and hydroxyl group content of the support surface. For example, the adsorption of CO on highly dispersed clusters of Rh71973-74 as well as Ru75 supported on yA1203 leads to tbe oxidative fragmentation of the clusters; the resulting surface species are metal subcarbonyls. When clusters of more noble metals (Pd76 or Pt77) on metal oxide or zeolite supports are brought in contact with CO, they aggregate to form larger clusters. The surface hydroxyl groups have been suggested to play a principal role in both cases. Thus it appears that the combination of iridium and the zeolite may be nearly unique in allowing the reversible decarbonylation and recarbonylation with retention of the cluster nuclearity. It may be that iridium is not too noble and sufficiently oxophilic to resist ready agglomeration or fragmentation, and the zeolite cages may help to stabilize the hexairidium clusters. Experimental Section Materials and Catalyst Synthesis. Synthesis of the zeolite-supported organometallicswas performed with samples in a Braun M E 150M drybox purged with N2 that recirculated through 02-and moisture-scavenging traps or on a Schlenk vacuum line that was purged with N2 (99.999%). The drybox was equipped with 0 2 and moisture detectors and the concentrations of these contaminants were
